Argon is a component of air that is present at slightly less than a 1% mole fraction. In FIG. 1, a cryogenic air separation system is illustrated for producing gaseous oxygen, gaseous nitrogen and liquid oxygen. Argon is also a product of the cryogenic air separation process and regulation of product argon impurities is critical to optimization of argon production and operations safety of the process. In the system of FIG. 1, air is first compressed to approximately 5-6 atmospheres in a compressor 10, purified and fed to a high pressure column 12 where the air feed undergoes a preliminary separation into liquid fractions of crude oxygen and substantially pure nitrogen. A portion of nitrogen outflow passes via a pipe 14 to a high pressure product gaseous nitrogen output 16. The remainder of nitrogen outflow is condensed in a condensor/reboiler, is subcooled in a heater exchanger 20, and is then provided as liquid reflux to the top of a low pressure column 24. The liquid oxygen fraction from high pressure column 12, comprising oxygen, argon and nitrogen, is fed via pipe 18 through a heat exchanger 20, a condenser 22 and into a side feed in a low pressure column 24. An inflow of externally supplied liquid nitrogen is also fed to low pressure column 24 via pipe 25. Due to the relative volatilities of nitrogen, argon and oxygen, argon accumulates in an intermediate stripping section of low pressure column 24 where it is withdrawn to form a feed fraction for a side arm argon column 26. Gaseous nitrogen is recovered from the top of low pressure column 24 and gaseous and liquid oxygen are recovered from the bottom thereof.
A stream of argon vapor is withdrawn from the top of argon column 26 and is condensed in a condenser 22. A fraction of the argon stream is withdrawn from condenser 22 and is discharged as a product argon stream at output 28. Further details of the operation of the system shown in FIG. 1 can be found in U.S. Pat. No. 5,313,800 to Howard et al., assigned to the same Assignee as this application. The disclosure of the Howard et al. patent is incorporated herein by reference.
Regulation of product argon impurity is critical to optimization of argon production and is accomplished by action of process control computer 30 which receives as inputs, multiple measured values from various connected analyzers (A) and issues process control and regulation commands to control instrumentalities (not shown).
Process measurements from low pressure column 24, when properly evaluated, provide information about a subsequent dynamic response of product argon impurity at outflow 28. Certain measurements at low pressure column 24 are partially redundant to measurements of the product argon stream 28 in that they provide similar information about what changes are occurring in the process, although at different times. As these measurements are partially redundant and because generally no manipulatable variable or combination of variables exists for producing independent changes in these measurements, such measured properties cannot be independently controlled.
A prior art method for dealing with product argon nitrogen impurity control and involves a cascade control procedure is shown in FIG. 2 where product argon nitrogen content is cascaded to the control of argon column feed argon content. Nitrogen content is controlled by manipulation of an argon content set point. Argon content, by contrast, is controlled by manipulation of low pressure column gaseous oxygen product flow. This arrangement enables a slower responding product argon nitrogen content controller to provide a calculated set point to a faster responding feed argon content controller.
In specific, the measured product argon nitrogen content is compared against minimum and maximum targets (decision box 40) and if between those targets, the argon nitrogen content set point is set equal to the current measured argon nitrogen measured content (box 42). If the argon nitrogen content is not within the targets, the argon nitrogen set point is set equal to the nearest target limit value (box 43). The procedure then moves to a control calculation (box 44), based upon the selected argon nitrogen set point and the current value, as measured. The output of this control calculation is an argon column feed argon set point that will drive the argon nitrogen content to its set point.
The calculated feed argon content set point is then fed to a feed argon content control calculation procedure (box 46) which also has, as an input, the measured argon column feed argon content. The control calculation output is an oxygen flow change value to enable an alteration of the nitrogen content in the argon stream. A further compensation occurs in response to a measured column air flow change (box 48). The oxygen flow change and any compensation required as a result of a column airflow change are summed in summer 49. The output is a calculated output flow set point change to an oxygen flow controller.
As shown in FIG. 2, the control procedure comprises a first loop including boxes 40, 42, 43 and 44 which, together, provide a feed argon content set point, and a second loop which, based upon the argon content set point, calculates an oxygen flow set point for product argon nitrogen content control. The cascade control shown in FIG. 2 depends upon the second loop achieving the target calculated by the first loop well within the process response time of the first loop. If the first loop makes adjustments before the response of the second loop is completed, the cascaded controllers fight each other and degrade the performance of the system.
As indicated above, a control action that is responsive to argon column feed argon content is preferred as that variable responds to process upsets significantly sooner than product argon nitrogen content. However, as the two quantities, i.e., product argon nitrogen content and argon column feed argon content, are essentially dependent variables, independent control thereof is not possible.
Accordingly, it is an object of this invention to provide an improved control procedure which enables a more rapid response of a system being controlled to process upsets and set point limit changes.
It is another object of this invention to employ measured changes in an intermediate product of a process to predict changes in an output product measurement.
It is still another object of this invention to provide an improved air separation control process wherein argon stream nitrogen content is more precisely controlled as a result of prediction estimates derived from measurements of argon column feed argon content or product oxygen impurities.